Kicker—Tap an untapped Vampire you control. (You may tap a Vampire you control in addition to any other costs as you cast this spell.) Target opponent loses half his or her life, rounded up. If Blood Tribute was kicked, you gain life equal to the life lost this way.
Official Rulings for Blood Tribute
10/1/2009 : You can tap a creature that hasn't been under your control since your most recent turn began to pay the kicker cost.